The present invention relates to a cipher system used for communication between a host computer and terminal equipments, and more particularly, to a cipher system with a closed cipher procedure which makes it possible to realize a system architecture without opening the cipher procedure algorithm to supervisors at the terminal equipments.
Conventional techniques regarding cipher systems of computer networks or the like, are known disclosed, for example, in a cipher algorithm "DES" enacted by Department of Commerce of US Government in 1977 or in a publication "Nikkei Communications", June 22, 1985, pp. 58 to 66. According to such conventional techniques, an identical cipher process program is provided to both a host computer and a terminal equipment to encipher data at a sending party and decipher the data at a receiving party to obtain a plain text. In such a cipher process system, both the host computer and the terminal equipment carry out the same cipher process. Therefore, in developing a terminal equipment or in other cases, the cipher procedure algorithm developed at the host computer is made open to the manufacturer or supervisor of the terminal equipment to prepare a cipher process program at the terminal equipment.
If the manufacturer or supervisor is the same for both the host computer and the terminal equipment, the above-described conventional technique poses no problem since the cipher procedure algorithm is not made open to the third party. Due to recent progress in networking, it has become possible to communicate between equipments of different makers, between equipments under different supervisors, and between equipments of different business fields, within a single network constructed of a plurality of host computers and terminal equipments. If a conventional cipher system technique is applied to such a system, the cipher procedure algorithm must be made open to associated parties under communication, thus posing a problem of a difficulty to keep secret the cipher procedure algorithm. Although various systems to solve such a problem and avoid unauthorized utilization of a computer network have been studied, a perfect solution has been not given to date.